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A Comprehensive Guide to Kotlin Null Safety

6 April 2023

A Comprehensive Guide to Kotlin Null Safety

This article is brought to you by JBI Training, the UK's leading technology training provider.   Learn more about JBI's Tech training courses including a range of Kotlin training courses. Kotlin Best Practices & Kotlin Beyond the Basics

 

A Comprehensive Guide to Kotlin Null Safety

Kotlin is a modern programming language that has gained a lot of popularity among developers because of its simplicity and ease of use. One of the most important features of Kotlin is its null safety, which helps developers avoid null pointer exceptions, a common error in programming. In this guide, we will explore Kotlin null safety and provide step-by-step instructions on how to use it effectively.

What is Null Safety?

Null safety is a feature of Kotlin that allows developers to write code that is more robust and less prone to errors. In Kotlin, null safety is built into the language, meaning that it is impossible to assign null to a non-nullable variable. This means that developers can avoid null pointer exceptions, which occur when a program tries to access a null value.

Null safety is achieved through the use of nullable and non-nullable types. In Kotlin, every type is non-nullable by default, meaning that it cannot hold a null value. However, it is possible to declare a variable as nullable by adding a question mark to the end of its type.

For example, the following code declares a non-nullable variable:


 
kotlin

 
val x: Int = 5

If we try to assign null to x, we will get a compilation error:


 
csharp

 
x = null // Compilation error: null can not be a value of a non-null type Int

To declare a nullable variable, we add a question mark to the type:


 
kotlin

 
val y: Int? = null

Now we can assign null to y without getting a compilation error:


 
csharp

 
y = null // OK

Using the Safe Call Operator

One of the most common use cases for null safety is checking for null values when accessing properties or calling methods. In Kotlin, we use the safe call operator ?. to do this.

The safe call operator is used to access a property or call a method on an object that may be null. If the object is null, the safe call operator returns null instead of throwing a null pointer exception.

For example, consider the following code:


 
kotlin

 
val s: String? = null val length = s?.length

In this code, we declare a nullable string s and then use the safe call operator to get its length. Since s is null, the length variable will also be null, and no null pointer exception will be thrown.

Using the Elvis Operator

Another useful operator in Kotlin is the Elvis operator ?:. The Elvis operator is used to provide a default value when a variable is null.

For example, consider the following code:


 
kotlin

 
val s: String? = null val length = s?.length ?: 0

In this code, we use the safe call operator to get the length of s. If s is null, the Elvis operator returns 0 as a default value for length.

Using the Not-Null Assertion Operator

In some cases, we may know that a variable is not null, but the compiler may not be able to infer this. In these cases, we can use the not-null assertion operator !! to tell the compiler that the variable is not null.

For example, consider the following code:


 
kotlin

 
val s: String? = "Hello" val length = s!!.length

In this code, we use the not-null assertion operator to tell the compiler that s is not null. If s is null, a null pointer exception will be thrown at runtime.

Best Practices for Null Safety

When using null safety in Kotlin, it is important to follow some best practices to avoid common mistakes and ensure that your code is easy to understand and maintain. Here are some

best practices for null safety in Kotlin:

  1. Always use nullable types when a variable may hold a null value. This will help you avoid null pointer exceptions.

  2. Use the safe call operator ?. whenever you access a property or call a method on an object that may be null.

  3. Use the Elvis operator ?: to provide default values for nullable variables.

  4. Use the not-null assertion operator !! sparingly and only when you are absolutely certain that a variable is not null.

  5. Avoid using !! on the left side of an assignment statement. This can result in hard-to-debug null pointer exceptions.

  6. Use type inference to avoid redundant null checks. For example, instead of writing if (x != null) { println(x.length) }, you can write x?.let { println(it.length) }.

  7. Use the @Nullable and @NotNull annotations to help the compiler detect nullability errors.

Conclusion

Null safety is an important feature of Kotlin that helps developers write more robust and less error-prone code. By using nullable and non-nullable types, safe call operators, Elvis operators, and not-null assertion operators, developers can avoid null pointer exceptions and improve the quality of their code.

In this guide, we provided an overview of Kotlin null safety and demonstrated how to use it effectively. We also provided some best practices for using null safety in Kotlin. With these tools and techniques, you can write Kotlin code that is more reliable, more maintainable, and less prone to errors.

Based on the topics covered in this article, here are some relevant tech training courses from JBI Training:

  •  Kotlin: Develop Android Apps or Java Backend applications with Kotlin - Reach A Growing And Powerful Mobile Web Audience
  • Kotlin Best Practices: Gain highly effective skills for developing in Kotlin for Android, and Java Backends
  • Kotlin Beyond the Basics: Develop Android Apps or Java Backend applications with Kotlin - Reach A Growing And Powerful Mobile Web Audience 

Here are some official documentation and links for Kotlin:

  1. Kotlin Official Website: https://kotlinlang.org/ - This is the official website of Kotlin where you can find everything related to Kotlin including documentation, tutorials, news, and community resources.

  2. Kotlin Documentation: https://kotlinlang.org/docs/home.html - This is the official documentation of Kotlin which includes a comprehensive guide, tutorials, and references for Kotlin.

  3. Kotlin Programming Guide: https://kotlinlang.org/docs/home.html - This is a detailed guide to Kotlin programming that covers the basics of the language and advanced topics like coroutines, functional programming, and more.

  4. Kotlin Standard Library: https://kotlinlang.org/api/latest/jvm/stdlib/ - This is the official Kotlin Standard Library documentation which includes a list of classes and functions that are available in Kotlin out-of-the-box.

  5. Kotlin Style Guide: https://kotlinlang.org/docs/reference/coding-conventions.html - This is the official Kotlin Style Guide that provides a set of rules and conventions for writing Kotlin code.

  6. Kotlin Coroutines: https://kotlinlang.org/docs/coroutines-overview.html - This is the official Kotlin documentation for coroutines which is a lightweight concurrency framework for Kotlin.

  7. Kotlin Playground: https://play.kotlinlang.org/ - This is an online Kotlin playground where you can write, run, and share Kotlin code snippets.

  8. Kotlin Blog: https://blog.jetbrains.com/kotlin/ - This is the official Kotlin Blog where you can find news, updates, and articles related to Kotlin.

I hope you find these resources helpful in your Kotlin learning journey!

About the author: Daniel West
Tech Blogger & Researcher for JBI Training

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